The element chromium is an important and strategic metal in our modern society. It is used extensively in the production of stainless steels and other corrosion resistant applications such as chrome plating. Chromium is also used extensively in the chemical industry in producing chromium containing compounds for a variety of uses including leather tanning and paint pigments. Chromium in the form of chromite is used in the production of refractory bricks for lining high temperature furnaces and in the glassmaking and cement industries.
Chromium is usually derived from chromite ore which includes chromite, FeCr.sub.2 O.sub.4, as a primary constituent. Chromite ores typically include other oxides such as silicon dioxide, SiO.sub.2, magnesium oxide, MgO, and aluminum oxide, Al.sub.2 O.sub.3. Other oxides may also be present in varying quantities depending upon the specific chromite deposits in question.
Chromite ores are typically classified on the basis of their chromium to iron ratio, Cr/Fe ratio, and the concentration of chromium in the ore. The highest grade of chromites are those having chromium to iron ratios of greater than 2.0 and having a minimum of 46% to 48% Cr.sub.2 O.sub.3. This highest grade of chromites are typically referred to as metallurgical grade chromites. They occur in significant quantities in South Africa, Zimbabwe, and the USSR.
The next grade of chromite ores are typically called chemical and refractory grade chromites which usually have concentrations of Cr.sub.2 O.sub.3 in the range of 40 to 46 percent. Chromium to iron ratios typically range from 1.4 to 2.0 although chemical grade chromite ores sometimes contain large amounts of iron which can result in chromium to iron ratios as low as 1.0. Refractory grade chromites contain relatively large quantities of aluminum oxide Al.sub.2 O.sub.3.
Low grade chromites are those having chromite to iron ratios of less than approximately 1 and containing less than 40 percent Cr.sub.2 O.sub.3.
The United States does not have developed deposits of high grade chromite ore. Most of the chromite deposits in the U.S. are of relatively low grade and hence are currently uneconomical for use in producing chromium metal, higher grades of chromite, or ferro-chromium compounds used in the steel industry. The United States does have very significant deposits of relatively low grade chromite ores in coastal beach sands of southwest Oregon, the Stillwater complex of southcentral Montana, California, and Alaska. These low grade deposits have not been commercially developed because higher grade ores from foreign countries can be more economically refined. Because of the strategic importance of chromium and the large amount consumed by the U.S., it is apparent that a relatively cost effective method for upgrading or beneficiating these lower grade domestic ores is much desired.
Prior art processes have attempted to beneficiate these low grade chromite ores using a number of techniques. Oregon beach sand deposits of chromite have been gravity concentrated by tabling or by the use of Humphreys spirals. Flotation techniques have also been tried but have not been widely used due to low recoveries.
Work by the U.S. Bureau of Mines explored roasting chromite ores under strongly reducing conditions with carbon at temperatures near 1300.degree. C. This roasting process reduced the iron in the chromite ore which was then dissolved in dilute sulfuric acid with most of the chromium remaining because of its relative insolubility in the sulfuric acid solution.
Reduction of chromite ores using graphite have also been tested at temperatures in the approximate range of 1150.degree. to 1450.degree. C. Coal char was also used in lieu of graphite as the reductant. Smelting of chromite ores with subbituminous coal and char have also been tried.
Other reducing gases such as hydrogen, methane, natural gas, and carbon monoxide have been used to reduce the iron in chromite ores usually by roasting at elevated temperatures. Leaching of the resulted roasted ore using sulfuric acid at atmospheric pressures has also been tested.
Tests involving sulfating roasting and leaching with ammonium sulfate have been tried but not considered economical due to excessive losses of ammonia and a required crystallization of alum to separate iron from the chromium.
The closest prior art experimentation known was conducted by Dwight L. Harris and reported in the Transactions of the Society of Mining Engineers, September 1964 at pp. 267-281. The Harris study investigated many different reduction and leaching processes using carbon, chlorine, ammonium and sulfation reduction steps. Reduction with gaseous SO.sub.2 and SO.sub.3 in combination with air and in concentrations ranging from 10 to 50% sulfur oxides were conducted. The sulfation roasting of the chromite ores occurred at temperatures ranging between 900.degree. to 1340.degree. F. for roasting periods of approximately three to four hours. The chromite subjected to the sulfating roasts were subsequently leached in a solution of water with 5% by weight sulfuric acid. Harris concluded that sulfation roasting with sulfuric acid leaching was not economical. Harris also experimented with using ferric sulfate leaching of chromite ore concentrate which had previously been reduction roasted with coal.
It is the object of this invention to not only beneficiate low grade chromite ores using sulfur dioxide and air mixtures, but to also scrub sulfur dioxide from waste gases which are produced in great volume by many industrial processes and which are ordinarily vented. In many instances, the concentrations of SO.sub.2 which are emitted through such vents must necessarily be reduced in order to comply with environmental laws. Thus it appears that economic savings are possible by performing both functions simultaneously.
It is also an object of this invention to leach reacted chromite ores to thereby remove iron, magnesium, and other impurities from the ore to increase the chromium to iron ratio and the concentration of chromium in the resulting product.